function time_length(begin_unix, end_unix){
  var diff_unix= (end_unix - begin_unix)
  var h = parseInt(diff_unix/3600)
  var m = (parseInt(diff_unix/60))%60
  var s = parseInt(diff_unix%60)
  var time_len = (h>9?h:'0'+h)+':'+(m>9?m:'0'+m)+':'+(s>9?s:'0'+s)
  return time_len
}

module.exports={
  time_length
}